1. Field of the Invention
The present invention relates to a method and an apparatus for driving a display device such as an ac driven capacitive flat matrix display panel (hereinafter referred to as thin film EL display device), etc.
2. Description of the Prior Art
For example, a double insulation type (or three-layer structure) thin film EL element is formed in the following way.
As shown in FIG. 1, band-like transparent electrodes 2 are disposed in parallel to each other on a glass substrate 1. On this, a dielectric material 3a such as Y.sub.2 O.sub.3, Si.sub.3 N.sub.4, Al.sub.2 O.sub.3, etc., an EL material 4 composed of ZnS to which an active agent such as Mn, etc. is doped, and a dielectric material 3b such as Y.sub.2 O.sub.3, Si.sub.3 N.sub.4, TiO.sub.3, Al.sub.2 O.sub.3, etc. likewise as above, are laminated in turn to a film thickness of 500-10000 .ANG. by using a thin-film technique such as vacuum deposition or sputtering to make a three-layer structure. On this structure, band-like back electrodes 5 composed of Al (aluminum) are disposed in parallel to each other at direction of right angles to the transparent electrode 2.
In the thin film EL element, the EL material 4, sandwiched by dielectric materials 3a, 3b, is disposed between the electrodes. Thus, it can be regarded as a capacitive element in terms of an equivalent circuit. As is apparent from the voltage - brightness characteristics shown in FIG. 2, this thin-film EL element is driven by applying a relatively high voltage of about 200 V. This thin-film EL element has characteristics that it emits highly bright light by an ac field, and yet its service life is long.
In such a display device using a thin-film EL element as a display panel, the following method (so-called symmetrical driving method) is generally used as a suitable method for keeping the display quality of a thin-film EL element which is ac driven. One of either the transparent electrode 2 or the back electrode 5 is made to serve as a data side electrode, and the other is made to serve as a scanning side electrode. A gradation voltage is applied to the data side electrode according to gradation display data on the one hand. A writing voltage is applied to the scanning side electrode in the order of line. Further, the writing voltage completes a display of one frame in a first field that serves as one of the polarities to the data side electrode and a second field that serves as the other electrode.
This driving method causes a superimposing or offset effect of the writing voltage and a modulation voltage to occur at each picture element corresponding to an intersecting position of the data side electrode and the scanning side electrode among the above EL layers 4. The voltage substantially applied to the picture elements becomes more than a light emission threshold voltage or a value less than a light emission threshold voltage. This causes each picture element to show a state of light emission or non-light emission, and a predetermined display is obtained.
In such a display device, when the brightness of each picture element is displayed in a gradation display of several steps, a pulse width modulation method by which the pulse width of a modulation voltage applied to a data side electrode is changed according to gradation display data, or an amplitude modulation method by which the amplitude of a modulation voltage is changed according to gradation display data, is adopted.
In the case of gradation display by a pulse width modulation method, for example, in a field of N driving, a modulation voltage of the pulse width corresponding to the gradation to be displayed to the data side electrode, and a voltage of a waveform to which the amplitude of a modulation voltage is superimposed into the absolute value of the amplitude of a writing voltage, is applied to a picture element. Hence, the section corresponding to the pulse width of a modulation voltage is one that exceeds a light emission threshold voltage. That is, the area of a voltage (intensity) applied to a picture element increases as the pulse width of a modulation voltage becomes longer, and decreases as it becomes shorter. On the other hand, in a field of positive driving in which a writing voltage of positive polarity is applied to the scanning side electrode, a modulation voltage of a pulse width opposite to the gradation to be displayed (short pulse width for a highly bright gradation, long pulse width for a low bright gradation) is applied to the data side electrode, and a voltage of the waveform in which the amplitude of the writing voltage is subtracted by the amplitude of a modulation voltage, is applied to a picture element. Therefore, the section that the pulse width of the writing voltage is subtracted by the pulse width of a modulation voltage, is one that exceeds a light emission threshold voltage. That is, the area of a voltage (intensity) applied to a picture element decreases as the pulse width of a modulation voltage becomes longer, and increases as it becomes shorter.
As described above, with symmetrical driving such that display of one frame is completed by a field of negative driving and a field of positive driving, when gradation display is made, the relation of a pulse width of a modulation voltage must be inverted for the negative driving and the positive driving even if it is of the same gradation. In other words, gradation display data must be set opposite in the cases of negative driving and positive driving. This also applies to the case of an amplitude modulation method for a modulation voltage.
However, incorporating the function that converts the relation of the gradation to be displayed and gradation display data according to negative driving and positive driving as described above to peripheral circuits and peripheral devices of a display device, nullifies the wide use of those peripheral circuits and peripheral devices. So, the conversion function has generally been shared by a driving circuit that directly applies a modulation voltage to the data side electrode. Therefore, a problem of the a driving circuit being complicated has existed.